1. Field of the Invention
The present invention relates to a mounting structure for a plurality of electro-optical devices, and more particularly to a mounting structure for charge coupled devices (CCDs) in an image scanner or for LED-arrays in a printer.
2. Discussion of Related Art
In the art of image scanners using CCDs, reduction type systems and so-called contact type systems have been known. In a reduction type system, a line of a document to be scanned is imaged onto the CCD at a reduced scale, and the operative range of the CCD, i.e. the length of the line on the document to which the CCD is sensitive, is substantially larger than the length of the CCD itself. In a contact type system, the CCD is directly facing the document to be scanned and, accordingly, the sensitive range is substantially equal to the length of the CCD. In practice, the sensitive range is slightly smaller than the length of the CCD, because the CCD element has end portions which are not light-sensitive.
In commercially available CCDs, the number of light-sensitive elements which are sensitive to individual pixels is limited to approximately 5000. As a consequence, in both reduction type systems and contact type systems, the length of a line segment which can be scanned with a single CCD is along a line in such a manner that their operative ranges are consecutive. The CCDs must be carefully positioned in order to avoid the production of junction errors at the borders of the respective operative ranges when the scanned image is reproduced. However, since the CCDs and the substrate on which they are mounted are generally subject to different thermal expansion, for example because they are heated by a lamp used for illuminating the document, it is difficult to maintain a correct positional relationship between the CCDs.
In U.S. Pat. No. 4,849,820 there is disclosed a reduction type scanner in which two CCDs are mounted on a common base plate which is fixed only at one end, so that it may expand or contract in the case of temperature variations. The first CCD is directly mounted on the base plate, and the second CCD is mounted on a secondary support plate one end of which is fixed to the base plate in the vicinity of the free end of the latter. The base plate and the support plate consist of different materials, and the lengths of these plates are so adapted to the respective thermal expansion coefficients that the thermal expansions cancel each other and the position of the second CCD relative to the first one remains substantially unchanged.
However, this solution is not applicable in cases where the electro-optical devices, e.g. contact type CCDs, have a substantial length and the thermal expansion of the CCDs themselves must be taken into consideration. In such cases, if only two CCDs are employed, the CCDs may be mounted in a staggered configuration with overlapping end portions, so that their operative ranges are consecutive, and both CCDs may be fixed to the substrate in the vicinity of their adjacent end portions. If, however, a third CCD is to be added, thermal expansion becomes a serious problem.